Wideband Code Division Multiple Access Radio Access Network (WRAN) introduced High Speed Downlink Packet Access (HSDPA) in 3GPP Release 5 and High Speed Uplink Packet Access (HSUPA)/Enhanced Uplink (EUL) in 3GPP Release 6. High Speed Packet Access (HSPA) is the common term for both HSDPA and EUL. 3GPP specifications from Release 4 to Release 6 use a few fixed Medium Access Control-d (MAC-d) Packet Data Unit (PDU) lengths for HSPA. The Radio Link Control (RLC) transmission window limitation of 2,047 PDUs together with the rather long Round-Trip-Time (from the serving radio network controller to user equipment and back) gives a limited peak bitrate in cellular systems.
The introduction of Multiple Input, Multiple Output (MIMO) and/or 64 Quadrature Amplitude Modulation (QAM) may result in peak bitrates as high as 42 Megabits per second (Mbps). Longer MAC-d PDU lengths are needed for a higher HSDPA peak bitrate (assuming that the RLC window size of 2,047 is maintained). Using MAC-d PDUs that are too long causes limited coverage, as long as only an integer number of MAC-d PDUs is scheduled over the air-interface in one Transmission Time Interval (TTI), i.e., one MAC-d PDU is the smallest unit of data that can be transmitted in one TTI.
RLC Acknowledged Mode (AM) provides a structure for using flexible PDU lengths. In, for example, RLC AM (3GPP TS 25.322, RLC protocol specification), a flexible PDU length structure is defined. There is a possibility to configure several RLC PDU lengths, but header fields may restrict the de-facto number that can be used. For example, it is currently possible to use maximally 8 different MAC-d PDU lengths over HS-DSCH, where a MAC-d PDU includes an RLC PDU and optional MAC-d header. A completely new PDU length structure is, therefore, needed for optimal performance.
The current solution for High Speed Downlink Shared Channel (HS-DSCH) capacity allocation and the definition of the HS-DSCH DATA FRAME are not efficient for the flexible RLC (or flexible PDU length structure) solution. The HS-DSCH Data Frame bitrate cannot be controlled well using the current HS-DSCH Capacity Allocation Control Frame format. The current control frame format specifies that a given number of PDUs (HS-DSCH Credits) of given maximum length (Maximum MAC-d PDU Length) can be sent in a given interval (HS-DSCH Interval). Assuming a fixed MAC-d PDU length, it is easy to translate this format to octets within an interval, or to a bitrate. However, with the introduction of flexible RLC, each and every MAC-d PDU can be of different length. Thus, a PDU of one octet consumes a full credit, just like a PDU of 1,500 octets, and controlling the allowed number of octets per interval, or the allowed bitrate, becomes difficult.
The initial capacity of HS-DSCH data transfer is granted by the base station via the HS-DSCH Initial Capacity Allocation during the Radio Link Setup procedure, the Radio Link Reconfiguration procedure, or the Radio Link Addition procedure. During these procedures, the HS-DSCH Initial Capacity Allocation, which is sent by the base station to the controlling radio network controller, specifies the maximum MAC-d PDU length (Maximum MAC-d PDU Size) and the number of MAC-d PDUs (HS-DSCH Initial Window Size). The current interpretation of this HS-DSCH Initial Capacity Allocation is used for fixed MAC-d PDU lengths and is obviously not suitable for flexible RLC.
The current HS-DSCH DATA FRAME format does not support different MAC-d PDU lengths. Sending MAC-d PDUs of different length might become very inefficient (e.g., the transport network overhead can become very high), because a new DATA FRAME is needed for every PDU of different length. Moreover, in the current format, a 4-bit spare extension is inserted in front of every MAC-d PDU in the data frame, increasing the overhead significantly in the case of octet aligned PDUs, which is a common case. The current format cannot handle the flexible RLC approach (e.g., where a MAC-d PDU containing a 1,500 octets long Internet Protocol (IP) packet is to be sent). At the same time, the current MAC-d PDU length indicator assumes bit granularity, which is not needed if MAC-d PDUs become octet aligned with the removal of MAC-d multiplexing. Moreover, with the removal of MAC-d multiplexing, if the HS-DSCH DATA FRAME does not support some type of logical channel mapping, the number of transport network connections needed by some Radio Bearers might increase significantly (e.g., instead of one connection, four connections may be needed for a Signaling Radio Bearer (SRB)).